Among various methods of hard facing various kinds of structures, machine parts, instruments, etc. for prevention of wear, corrosion, high temperature oxidation, errosion etc., there is known a method of hard facing through use of what is called the self-fluxing alloy by means of spraying or build up welding.
Self-fluxing alloys comprise a base of nickel (Ni), nickel-chromium (Ni-Cr) or cobalt-chromium (Co-Cr) and additives of boron (B) and silicon (Si). Among such self-fluxing alloys nickel-base alloys containing 1 to 3% by weight of boron and 2.3 to 5% by weight of silicon are widely used because of their relatively good wear resistance, corrosion resistance and workability for hard facing.
However, the above-mentioned conventional self-fluxing nickel-base alloys have a disadvantage that when they have been applied onto a large piece of base metal or a base of a metal which has a thermal expansion coefficient greatly different from those of the alloys, craks or fissures will occur in the hard facing alloy layer under certain conditions of employment.
This disadvantage is caused by the presence of a quasi-binary eutectic structure of a Ni-solid solution+Ni.sub.3 B in the structure constituting the matrix of the conventional self-fluxing nickel-base alloys.
Since the Ni.sub.3 B in the quasi-binary eutectic structure of Ni-solid solution+Ni.sub.3 B is so brittle that this binary eutectic structure is the least tough and ductile in the matrix, cracks or fissures occur in the hard facing layer of the alloy under certain conditions of employment as mentioned above.
Moreover, although the above-mentioned self-fluxing nickel-base alloys have relatively high wear resistance and corrosion resistance, these characteristics have been found not sufficiently satisfactory under certain conditions of employment with much room left for improvement.
Besides the self-fluxing nickel-base alloys there are known cobalt-base alloys available for hard facing. These alloys are composed of 0.9 to 1.6% by weight of carbon, less than 0.5% by weight of manganese, 0.8 to 1.5% by weight of silicon, 26 to 29% by weight of chromium, 4 to 6% by weight of tungsten, and less than 3% by weight of iron, the balance being cobalt. The alloys have a hardness of 35 to 45 in Rockwell C scale and a Charpy impact value of 0.9 to 1.4 kgm/cm.sup.2. Even under such conditions as would cause cracks or fissures in the hard facing layer of the conventional self-fluxing nickel-base alloys, the cobalt-base alloys are less susceptible to cracks or fissures and have relatively high wear resistance.
However, when the cobalt-base alloys are used at the places such as nuclear plants, where they are exposed to radioactivity, Co.sup.60 that is an isotope having a long half-life is produced with a resulting danger of environmental pollution. Therefore, it is undesirable to use a cobalt-base alloy for hard facing of the seat of a valve used in, e.g., an atomic power plant, and there has been a demand for hard facing alloys which can replace the cobalt-base alloys.
In an effort to solve the above technical problems, the present inventors have studied the compositions of self-fluxing nickel-base alloys and conducted various experiments, with the following three conditions having been set as the basic conditions the alloys of this invention are to satisfy:
(1) To have a hardness (Rockwell C scale) of more than 35. PA1 (2) To have an impact value (Charpy impact) of more than 0.9 kgm/cm.sup.2. PA1 (3) That the value of the hardness multiplied by the impact values should exceed 45. (This value will be referred to as the HI value hereinafter). PA1 (1) A hard facing nickel-base alloy consisting of 0.05 to 1.5% by weight of boron, 3 to 7% by weight of silicon, 7.5 to 35% by weight of chromium and 0.05 to 1.5% by weight of carbon, the balance being substantially nickel, and the weight ratio of silicon to bron (Si/B) being more than 3.3. PA1 (2) The hard facing nickel-base alloy described in the above (1) further containing less than 30% by weight of iron. PA1 (3) The hard facing nickel-base alloy described in the above (1) further containing less than 5% by weight of tungsten. PA1 (4) The hard facing nickel-base alloy described in the above (1) further containing less than 30% by weight of iron and less than 5% by weight of tungsten. PA1 (5) The hard facing nickel-base alloy described in either of the above (1), (2), (3) and (4) further containing 0.1 to 3% by weight of tin. PA1 (6) The hard facing nickel-base alloy described in either of the above (1), (2), (3) and (4) further containing 0.1 to 3% by weight of tantalum. PA1 (7) The hard facing nickel-base alloy described in either of the above (1), (2), (3) and (4) further containing 0.1 to 3% by weight of tin and 0.1 to 3% by weight of tantalum. PA1 (i) The matrix is composed chiefly of three elements, that is, nickel, boron and silicon. Since the nickel here is a solid solution containing silicon, a small amount of boron, and chromium, iron, copper, molybdenum, tungsten, etc. to be described hereinafter, this nickel solid solution will be referred to as (Ni) in order to distinguish it from pure nickel. PA1 (ii) A portion of the added chromium is dissolved in (Ni) in the solid state so as to enter the matrix, and the remainder of the chromium is combined with the carbon added simultaneously to form chromium carbide, chiefly complex chromium carbide M.sub.7 C.sub.3 type wherein M represents chiefly chromium with small amounts of molybdenum, tungsten, nickel, iron, etc. and also with a portion of the boron to form chromium boride, chiefly complex chromium boride MB type wherein M is the same as the above. The partition coefficient of the chromium, that is, what parts of the chromium are distributed to the matrix and M.sub.7 C.sub.3, and the partition coefficient of the boron, that is, what parts thereof are distributed to the matrix and MB are not certain. PA1 (iii) Since solidification of the M.sub.7 C.sub.3 and MB phases from the liquid alloy occurs at sufficiently higher temperatures (1270.degree. C. to 1420.degree. C.) than the temperature range in which the matrix components solidify (960.degree. C. to 1200.degree. C. and 960.degree. C. to 1080.degree. C. for most of the matrix components), the M.sub.7 C.sub.3 and MB crystallize as the primary and secondary phases so as to be dispersed in the matrix before the solidifying temperatures of the matrix components are reached. Since the M.sub.7 C.sub.3 and MB are both hard, they will be called the hard crystals. PA1 (iv) The iron and copper are dissolved chiefly in the (Ni) in the matrix, and the molybdenum and tungsten chiefly in the M.sub.7 C.sub.3 or MB. PA1 (v) As described above, the conventional self-fluxing nickel-base alloys of the above-mentioned compositions have a microstructure comprising either (a) a matrix composed of the three elements, (Ni), boron and silicon, or (b) a matrix composed of the same three elements as in the above (a) with the hard crystals, chiefly M.sub.7 C.sub.3 and a small amount of MB being dispersed therein under the coexistence of a large amount of chromium and carbon.
The hard facing nickel-base alloys which satisfy the above conditions sufficiently satisfy the usual conditions under which the cobalt-base alloys are actually used in various applications.
One object of this invention is to provide hard facing nickel-base alloys which satisfy the above three conditions and which are tough and ductile and superior in both wear resistance and corrosion resistance and have good workability for hard facing, and in which craks or fissures are unlikely to occur in the hard facing layer.
Another object of this invention is to further increase the corrosion resistance of such nickel-base hard facing alloys as above-mentioned.